Star
A star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun, which provides daylight and appears as a disk rather than a point of light. Other stars appear as fixed points in the night sky due to their immense distances from our planet. These objects shine because thermonuclear fusion converts hydrogen into helium within their cores. This process releases energy that traverses the interior and radiates into outer space. Only about 4,000 of these stars are visible to the naked eye, all located within the Milky Way galaxy. The observable universe contains an estimated number of stars far exceeding the grains of sand on Earth.
Ancient Egyptian astronomers produced the oldest accurately dated star chart in 1534 BC. Babylonian astronomers compiled early catalogues during the late 2nd millennium BC under the Kassite Period. Greek astronomer Aristillus created the first star catalogue around 300 BC with help from Timocharis. Hipparchus later included 1,020 stars in his own catalogue used for Ptolemy's work. Chinese astronomers recorded SN 185, the first supernova observation in history, in 185 AD. Ali ibn Ridwan wrote about the brightest stellar event ever recorded, SN 1006, which occurred in 1006. Persian astronomer Abd al-Rahman al-Sufi published the Book of Fixed Stars in 964 containing observations of galaxies like Andromeda. Tycho Brahe identified new stars suggesting heavens were not immutable while Giordano Bruno proposed stars might have planets orbiting them in 1584.
Stars condense from regions of space called molecular clouds that contain mostly hydrogen and helium. One example is the Orion Nebula where massive stars illuminate surrounding gas creating H II regions. When a region reaches sufficient density to satisfy Jeans instability criteria it begins collapsing under its own gravity. Individual conglomerations form Bok globules as the cloud collapses and temperature rises. A protostar forms at the core when hydrostatic equilibrium is reached after approximately 10 million years for Sun-like stars. Early stars less than 0.5 solar masses are called T Tauri stars while those with greater mass become Herbig Ae/Be stars. These newly formed objects emit jets of gas along their axis of rotation reducing angular momentum. The period of gravitational contraction lasts up to 100 million years for red dwarfs before reaching the main sequence.
Most stars spend about 90% of their lifetimes fusing hydrogen into helium within their cores on the main sequence. The Sun has increased in luminosity by about 40% since reaching the main sequence 4.6 billion years ago. Massive stars consume fuel rapidly and live only millions of years compared to trillions for low-mass red dwarfs. Stars with at least 8 solar masses expand into red giants then fuse heavier elements like carbon and oxygen. When helium exhausts at the core of massive stars they contract and temperatures rise enough to fuse carbon. This process continues through neon burning, oxygen burning, and silicon burning stages until iron production begins. Iron nuclei are more tightly bound so fusion beyond this point does not release energy. The core suddenly collapses as electrons drive into protons forming neutrons and neutrinos in a burst of electron capture. A supernova explosion blows away outer layers leaving remnants such as neutron stars or black holes depending on initial mass.
The current stellar classification system originated in the early 20th century when stars were categorized from A to Q based on hydrogen line strength. Modern schemes order classifications by temperature ranging from type O which are very hot to M which allow molecules to form. Main sequences fall along a diagonal band when graphed according to absolute magnitude and spectral type. The Sun is classified as a G2V yellow dwarf of intermediate temperature and ordinary size. Each letter has ten subdivisions numbered zero to nine indicating decreasing temperature levels. Rare types include L and T classes that classify coldest low-mass stars and brown dwarfs. White dwarf stars have their own class beginning with D further subdivided into DA DB DC DO DZ and DQ. Surface temperatures range from over 33,000 K for Zeta Ophiuchi down to 2,600, 3,850 K for Proxima Centauri. Astronomers determine properties like metallicity and rotational velocity using these spectral characteristics combined with luminosity measurements.
Around half of Sun-like stars form in multiple systems influencing phenomena like novae and supernovae formation. Binary star evolution differs significantly from single stars of equal mass due to gravitational interactions. When any star expands to become a red giant it may overflow its Roche lobe allowing material transfer to the companion. This yields phenomena including contact binaries common-envelope binaries cataclysmic variables blue stragglers and Type Ia supernovae. Mass transfer leads to cases such as the Algol paradox where the most evolved star is least massive. Systems of three or more stars exist but hierarchical binary sets ensure orbital stability. In dense regions like globular cluster cores collisions produce abnormal stars called blue stragglers. These objects have higher surface temperatures appearing bluer than main sequence turnoff points in clusters. Studies suggest all stars initially formed as binaries though some later split leaving single stars behind.
Stellar nucleosynthesis creates almost all naturally occurring chemical elements heavier than lithium within stars or their remnants. Massive stars explode as supernovae returning chemically enriched material to the interstellar medium for recycling into new generations. The portion of heavy elements measured as iron content indicates likelihood of planetary system formation. The dwarf HE1327-2326 has only one two-hundred-thousandth the iron content of the Sun while mu Leonis has nearly double solar abundance. Planetary nebulae ejected during asymptotic giant branch phases enrich general interstellar medium with carbon and oxygen. Future generations of stars form from this star stuff created by past stellar deaths. Population III stars likely existed in very early universe starting production of elements needed for planet and life formation. Stars today compose about 71% hydrogen and 27% helium by mass with small fractions of heavier elements. Older population II stars possess substantially less metallicity than younger population I stars due to molecular cloud composition differences over time.
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Common questions
What is a star and how does it produce light?
A star is a luminous spheroid of plasma held together by self-gravity. These objects shine because thermonuclear fusion converts hydrogen into helium within their cores, releasing energy that radiates into outer space.
When was the oldest accurately dated star chart created by ancient Egyptian astronomers?
Ancient Egyptian astronomers produced the oldest accurately dated star chart in 1534 BC. Babylonian astronomers compiled early catalogues during the late 2nd millennium BC under the Kassite Period.
How long does it take for a Sun-like star to form from a molecular cloud?
A protostar forms at the core when hydrostatic equilibrium is reached after approximately 10 million years for Sun-like stars. The period of gravitational contraction lasts up to 100 million years for red dwarfs before reaching the main sequence.
Why do massive stars end their lives as supernovae instead of white dwarfs?
Stars with at least 8 solar masses expand into red giants then fuse heavier elements until iron production begins. Iron nuclei are more tightly bound so fusion beyond this point does not release energy and the core suddenly collapses causing a supernova explosion.
What is the current stellar classification system used by astronomers today?
The modern scheme orders classifications by temperature ranging from type O which are very hot to M which allow molecules to form. Each letter has ten subdivisions numbered zero to nine indicating decreasing temperature levels and rare types include L and T classes that classify coldest low-mass stars.